The present invention relates to a heat-resistant laminate showing excellent gas-barrier property and an oilresistance at high temperature, and capable of enduring retort treatment at high temperature. More particularly, the present invention relates to a heat-resistant laminate film comprising outer layers of polypropylene, a layer of a vinylidene chloride copolymer and two adhesive layers of heat-resistant polyolefin and thermoplastic polyurethane, disposed between any of the above layers.
Recently, gas impermeable laminates capable of enduring boiling or retort treatment have been used in the field of food packaging. However, as the conditions for the treatment have become more and more severe, it has been demanded to dispose therebetween, in such laminates, adhesive layers capable of retaining good bonding strength at high temperature. However, many of conventional adhesives including ethylene--vinyl acetate copolymers are lack of the heat-resistance and accordingly, laminates having adhesive layers showing higher heat-resistance have been desired. As a laminate having the adhesive layers showing such heat-resistance Japanese Patent Application Laid-Open No. 54-30272 (1979) discloses a laminate film comprising outer layers of a polyolefin, a layer of vinylidene chloride synthetic resin and specific adhesive layers. However, the disclosed laminate film does not always exhibit good adhesive property under severe conditions such as retort treatment at a high temperature, for example, more than 120.degree. C. for a long time and accordingly, gas-impermeable laminates showing excellent adhesive property and heat-resistance have been demanded.
As the result of a study, the present inventors have found that a heat-resistant laminate film showing excellent gas-barrier property and an oil-resistance at high severe temperature, and having sufficiently practical interlaminar adhesive strength which is maintained even after the retort treatment for a long time at a temperature of more than 120.degree. C. and is increased with elapse of time, can be obtained by disposing two specific adhesive layers between heat-resistant resin layers as the outer layers and vinylidene chloride copolymer as the gas-barrier layer, that is, by disposing two adhesive layers comprising a layer of polyolefin modified by acid and a layer of thermoplastic polyurethane between the outer layers and the gas-barrier layer, and have attained the present invention on the basis of the above-mentioned finding.